Abstract: In order to explore the effect of delignification under different organic solvent-assisted alkaline hydrogen peroxide treatment, the composition changes of poplar mechanical pulp after lignin removal by ethanol alkaline hydrogen peroxide(EAHP) and dioxane alkaline hydrogen peroxide(DAHP) were investigated using the alkaline hydrogen peroxide system(AHP) as a reference. The chemical structure, molecular weight, and hydroxyl content of alkali lignin(AL) and alkaline hydrogen peroxide lignin(AHPL), ethanol alkaline hydrogen peroxide lignin(EAHPL) and dioxane alkaline hydrogen peroxide lignin(DAHPL) were characterized by high-performance liquid chromatography(HPLC), Fourier transforms infrared spectroscopy(FT-IR), gel-permeation chromatography(GPC), and ³¹P nuclear magnetic resonance(³¹P NMR). The results showed that the three reaction systems led to the varying degradation degrees of the material yield and the dissolution of a large number of hemicellulose. Compared with AHP and DAHP, EAHP was more conducive to preserve cellulose and dissolve more lignin, and the highest lignin removal rate reached 65%. Compared with AL, AHP, EAHPL and DAHPL possessed basically the same infrared spectral peak type, both belonging to G-S type. The minimum M_n of the prepared lignin was 4 672 g/mol with low polydispersity and low phenolic hydroxyl content, while it had higher aliphatic hydroxyl group and carboxylic acid hydroxyl group contents, with the highest contents of 14.95 and 1.25 mmol/g, respectively.